Telegraph switching system



March 15, 1949. o. BLYHOLDER TELEGRAPH SWITCHING SYSTEM 2 Sheeis$heet 1Filed Aug. 14, 1944 NUN JUL/E2770]? flr/andaB/y/w/ier March 15, 1949. o.BLYHOLDER 2,464,371

TELEGRAPH SWITCHING SY STEM 2 Sheets-Sheet 2 4 4 9 1 MW NIHI? g m j u k4 w v n u x F 33G 8W5 \I I Patented Mar. 15, 1949 iJNlTED STATES PATENTOFFICE TELEGRAPH SWITCHING SYSTEM Application August 14, 1944, SerialNo. 549,339

9 Claims.

This invention relates to telegraph switching systems. Its generalobject is to provide a telegraph switching system affording improvedfeatures of operation.

GENERAL DESCRIPTION The invention is illustrated as embodied in atelegraph switching system employing the socalled hub repeatingarrangement wherein two or more lines may be interconnected through acommon hub conductor having individual spoke conductors for therespective interconnected lines, and in which a two-way telegraphrepeater is interposed between each individual line and its associatedspoke conductor. In the hub repeating system as previously used, eachrepeater includes a receiving relay controlled over its associated lineto transmit signals over its spoke to the common hub, and a sendingrelay controlled over the common hub by way of the associated spoke totransmit signals to the line. In each repeater, the local spokeconductor is connected to the inductive winding of the associatedsending relay so long as the receiving relay is in its normal (marking)position, and is disconnected from such windin and transferred to asource of spacing current when the armature of the receiving relayleaves its marking contact and engages its spacing contact. Thereupon,spacing current flows over the associated spoke conductor to the commonhub, and thence over the spoke conductors of the interconnectedrepeaters to the windings of the sending relay of each. Patent 2,056,277issued to F. S. Kinkead et a1. October 6, 1936, shows a system operatingas above outlined. Patent 2,069,223 issued to G. C. Cummings February 2,1937, provides special arrangements for minimizing the detrimentalsparking and heating which occurs at the spacing contact of thereceiving relay in a repeater transmitting over the hub system to anumber of interconnected lines. Patent 2,077,917 issued to G. C.Cummings April 20, 1937, provides a hub-interconnecting repeater forinterconnecting two hub systems to reduce injury to the spacing contactof the receiving relay of a transmitting repeater.

In view of the foregoing, it is a specific object of the invention toprovide a telegraph repeating system in which a large number of linesmay be interconnected in a hub system without overloading the spacingcontact of the receiving relay of a transmitting repeater. This objectis attained in the present disclosure by replacing the sending relay ofeach repeater by a vacuum tube Whose control grid occupies the circuitposition occupied heretofore by the winding of the sending relay. Sincethe control grid of a vacuum tube, when properly biased, draws nocurrent, the only load on the spacing contact is represented by the gridresistor at each receiving repeater. Besides being of very highresistance, a grid resistor is, or may be, substantially non-inductive.Consequently, the required current fiow per line is greatly, reduced andis by way of a substantially non-inductive circuit. Both of theseeffects com-- bine to reduce the tendency to injure the spacing contactof the receiving relay in the transmitting repeater.

A further object is to provide a telegraph switching system in which thelines have ordinary appearances at the switchboard and in which reliableprovisions are made for signalling the switchboard operator over anyline when connection or disconnection is desired.

Other objects and features of the invention, more or less incidental tothe foregoing, will appear hereinafter.

THE DRAWINGS The accompanying drawings, comprising Figs. 1 to 5, showsufficient of the apparatus incorporated in a system embodying theinvention to enable the invention to be understood.

Fig. 1 shows one embodiment of the invention;

Fig. 2 shows a modification of the equipment associated with line L1 ofFig. 1;

Fig. 3 shows a second modification of such equipment;

Figs. 3A and 3B show modifications of Fig. 3;

Fig. 4 shows a power-supply unit suitable for the system of Figs. 1 and2; and

Fig. 5 shows a simplified power-supply arrangement for use in the systemof Figs. 3, 3A, and 3B.

FIGURE 1 Fig. 1 shows three subscriber lines of a system including anydesired number of such lines; first and second lines L1 and L2, and thelast line LL. These lines serve stations S1, S2, and SL respectively,shown diagrammatically as teletypewrite'r stations. The illustratedlines are provided with similar two-way repeaters R1, R2, andRLrespectively, repeater R1 being shown in full. Spoke conductors SP1,SP2, and SPL connect the illustrated repeaters respectively with theirswitchboard jacks, J1, J2, and JL, with which the jacks J 1, J2, and JL,are respectively multiplied, enabling patching cords such as PCl, P02,and P03 to be used to interconnect the spoke condoctors in any desiredarrangement of pairs and hubs.

Each repeater contains a receiving relay, such as RR of repeater RI. Thereceiving relay RR may be of any desired form of polarized relay, beingillustrated as of the type in which polarized armature passes axiallythrough the winding space. Such a relay may be adjusted to assume aneutral position with no current flowing through any of its threewindings. The armature of relay ER is normally held in marking position(against contact M) by the right-hand or marking winding, energized in acircuit including the positive pole of the current supply and theassociated regulating resistor. The righthand or marking winding may beeliminated if relay RR is biased to marking position, such asmagnetically or by spring action. The circuit of line Ll includes thetwo conductors thereof in series with the equipment at station Si. Itmay be considered as originating at the positive pole of thecurrent-supply source, and the normal current-flow thereover iscontrolled by the associated adjustable resistor. The return path forline current is through the left-hand or line wind- 7 ing of receivingrelay RR, and the plate of sending tube ST, to ground through thegrounded cathode element thereof. Assuming that the three windings ofrelay RR are equivalent, the

holdin the armature of the relay in markingw vertical arrows on 'thewindings in question are directed downwardly.

The magnetic effect of the current through the left-hand or line windingof receiving relay RR is neutralized by the opposed current flow ofsubstantially equal strength through middle or balance winding of therelay. The balance Winding is included in what may be termed anartificial-line circuit, which includes the associated adjustableresistor connected between such winding and the positive pole of thesupply source. The return path for the artificial line is through theplate of the balance tube BT, to ground through the grounded cathodeelement thereof.

Each of the tubes ST and ET is indicated as a tetrode, having itscontrol and screen grids in the usual relative positions. .Each screengrid is supplied with positive potential from the supply source throughthe associated individual resistor. The control grids of tubes ST and BTare both connected to the marking contact M of receiving relay RR, andare normally supplied with ground potential through the associated gridresistor GR, permitting a normal flow of current through both. A tubesuch as the commercially available 6L6 beam-power tube is satisfactoryat ST and BT,

using a positive supply potential of 250 volts, for

example.

Lines Ll to LL are provided with line circuits L0! to LCL respectively,of which the line circuit LC! is shown in full. Line circuit LC! iscontrolled over the associated spoke conductor SPI through the grid ofline tube LT. Tube LT, as

in excess of one second, for example, and to light the disconnect lampD! responsive to a break in the associated line in excess of liveseconds, for example, at a time when a connection with the line existsthrough the switchboard. Line circuit LCI includes line relay 2,normally operated through the plate-cathode circuit of line tube LT byhalf-wave-rectified current from relay terminal R, supplied withalternating current of commercial frequency and suitable voltage, as bysecondary winding 26 of the power-supply transformer 28 (Fig. 4). Relay2 may be rendered slow-restoring and substantially non-inductive by theusual copper sleeve surrounding its magnetic core and lying beneath itswinding, as is intended to be illustrated conventionally. Relay 2 isarranged to control the potential of the grid of timing tube TT bynormally maintaining a cutoff bias on such grid, across condenser E, andby discharging this biasing potential through resistor l for aone-second delay, and through resistors i and 8 for a five-second delay.As will hereinafter appear, timing tube TT controls one or another ofthe call and disconnect relays 5 and 4, according to the operated orrestored condition of transfer relay 3, controlled from either of theassociated jacks J l and J I. Relays 3 to 5 are supplied with currentfrom the associated relay terminals R, supplied through theabove-mentioned secondary winding 24 (Fig. 4). Each such relay ispreferably a shaded-pole relay which remains operated steadily whenenergized by alternating, pulsating, or direct current.

The switchboard operator is supplied with a telegraph station SO, alsoillustrated as a teletypewriter station, controlled over the operatorsline L0. This line terminates in the operators repeater R0, exactlysimilar to repeater Rl. Repeater R0 is connected with the operator'sswitchboard jack J O by the spoke conductor SPO.

FIGURE 2 winding in series with line Li and the other well as the othertriodes shown herein, may be of v the commercially available 605 type,using the lamp Cl at the switchboard responsive to a prolonged openingof line Ll (as by break key BK!) winding in series with the artificialor balance line of the repeater Ri. In the arrangement of Fig. 2, thetransmission of a disconnect signal from a connected line causes thelighting of the disconnect lamp associated with that line only, whereasin Fig. l the disconnect lamps light at all interconnected linesresponsive to a disconnect break at any one of them.

FIGURE 3 Fig. 3 shows a further modification of the equipment of line Ll(Fig. 1), illustrating what may be termed a uniform-current system. Thearrangement in Fig. 3 is such that current flows alternately in thesubscriber line and in the associated artificial or balance line,thereby simplifying the power-supply requirements, in addition tohalving the simultaneous or demand current for the several natural andartificial lines, as will be explained more in detail hereinafter. Fig.3 employs the additional tube PI, used as a phase-inverter triodeeffective to transmit a potential of the opposite sign to the controlgrid of balance 5.. tube B'I' when a potential of a given sign isimpressed on the control grid of the sending tube ST.

FIGURES 3A AND 33 Fig. 3A is a direct modification of Fig. 3 whereinsending tube ST is employed also as a phaseinverter tube with respect tobalance tube BT, rendering the use of the separate phase-inverter tubePI of Fig. 3 unnecessary.

Fig. 3B is similar to Fig. 3A, except that balance tube ET is used asthe phase inverter instead of tube ST.

FIGURE 4 Fig. 4 shows a power-supply arrangement suitable for the systemof Figs. 1 and 2 with its widely varying demands for plate current.Power is supplied through transformer 20, having primary winding 2|connected to a suitable source of commercial power. Secondary winding22, having its mid-point grounded, supplies alternating potential torectifier 25, through which the desired plate-supply and screen supplyvoltage is delivered to the associated positive output terminal. Astabilizing resistor 21 may be bridged between the positive outputterminal and the common ground lead, as illustrated. Secondary winding23 supplies alternating potential which is rectified by rectifier 26 tosupply a negative biasing potential to the associated negative outputlead. 28 is a stabilizing resistor for the biasing potential.

Secondary winding 24 has its lower terminal grounded, and the upperterminal brought out to terminal point R, used to supply operating potential to indicated ones of the relays of the line circuits such asLCl. An intermediate tap of transformer 24 is connected to terminal H tosupply heater current to the heater elements of the several vacuumtubes, and to supply lighting current for signal lamps such as Cl andDi.

FIGURE Fig. 5 shows a simplified modification of the power-supplyarrangement of Fig. 4 suitable for use with the system of Figs. 3 and3A, but which would very likely be unsatisfactory for the system ofFigs. 1 and 2 because of their Widely varying demands for plate current.The arrangement of DETAILED DESCRIPTION The disclosure having beendescribed generally, a detailed description of the operation of thesystem will now be given.

A. FIGURE 1 The system of Fig.1 will be considered first.

A1. Calling the operator Assuming that the subscriber at substation SI(Fig. 1) desires a connection, he opens the associated line Li, as bybreak key BKi, for an interval somewhat in excess of one second, andthen recloses the line. When the line is opened, the flow of linecurrent ceases, deenergizing the lefthand or line winding of receivingrelay RR. Thereupon, relay RR operates its armature from engagement withits marking contact M into engagement with its spacing contact S, byvirtue of the fact that the more powerful current in the middle orbalance winding then overcomes the current in the right-hand or markingwinding. Spacing current from the negative biasing terminal of thepower-supply arrangement of Fig. 4, is thereby impressed on theassociated spoke conductor SPl.

Since spoke conductor SP1 is normally disconnected at the jacks J I andJ l, the placing of the negative spacing potential thereon has no effectexcept in the line circuit LCl, where it blocks current flow throughline tube LT. The normally operated line relay 2 thereupon restores,disconnecting the grid of timer tube TT (and the upper terminal oftiming condenser t) from the source of biasing potential and connects itthrough the adjustable one-second resistor l, and contacts 3 of transferrelay 3, to ground. At the end of about one second, the normal negativecharge of grid condenser E is sufficiently dissipated through resistor lto render the control grid of timing tube TT sufficiently positive toenable plate current of relay-operating value to flow therethrough. Callrelay 5 thereupon operates in a circuit from ground, through the cathodeand plate of tube TT, armature I of transfer relay 3 and its backcontact, and thence through the winding of call relay 5 to relayterminal R, supplied with alternating potential by way of terminal R ofFig. 4. The current which flows through relay 5 at this time ispulsating because of the half-wave rectifying action of tube TT. Uponoperating, relay 5 closes a local self-lockingv circuit at its contactsI, to ground through the back contact of armature 2 of transfer relay 3.This self-locking circuit may include the illustrated current-limitingresistor. At its contacts 2, relay 5 closes a lighting circuit for calllamp Cl.

When the circuit of line L! is reclosed, the resumption of current flowthrough the line winding of receiving relay RR again neutralizes theeffect of current flow through the balance winding of the relay,permitting the right-hand or bias winding of the relay to return thearmature from spacing position to marking position. When this occurs,positive (ground) potential, obtained through grid resistor GR in therepeater RI is again applied to the grid of line tube LT in line circuitLCl, whereupon current flow is resumed-through tube LT and line relay 2.Relay 2 thereupon reoperates and disconnects the grid of timing tube TTfrom ground (cathode) potential by way of resistor l and transfers it tonegative biasing potential. When this occurs, condenser 6 again chargesto a negative value and the grid of tube TT becomes negative,terminating the flow of plate current therethrough. The resistorincluded in the lead extending to the front contact of relay 2 is toprevent momentary lowering of the biasing potential on the common supplylead by a sudden inrush of charging current to condenser 6. Thecessation of the flow of plate current through tube TT does not causecall relay 5 to restore because of the above-noted self-locking circuitof relay 5 through its contacts I. Only alternating current now flowsthrough the winding of relay 5, but this relay is able to remainoperated steadily when supplied only .with alternating current, aspreviously noted.

A2. Answering Upon noting the lighted condition of call lamp Cl, theoperator may insert one plug of an idle patching cord, such as PC3, intothe jack J I (or the jack J I), and insert the other plug of suchpatching cord into the operators jack JO. By this operation, spokeconductor SPI is connected electrically with spoke conductor SPO, theconcerned single-conductor patching cord serving as the common hubbetween the two spokes. With the repeaters RI and R both in markingcondition, both spokes are at ground potential, wherefore there is noimmediate interchange of current over the interconnection.

As a result of the insertion of a connecting plug into either of thejacks J l or J I, the upper contacts of such jack connect ground to thelower terminal of transfer relay 3, operating relay 3 in an obviouscircuit. Armature i of relay 3 disconnects the plate of tube TT fromcall relay and transfers it to relay 4. Armature 2 of relay 3disconnects ground from the self-locking circuit of call relay 5 andprepares a self-locking circuit for disconnect relay 4. Call relay '5now restores, thereby extinguishing call lamp Cl. Armature 3 of transferrela 3 removes the shunt normally existing around resistor 8 so as toincrease the effective discharge time of grid condenser 6 from about onesecond to about five seconds, thereby insuring that no likelycombination of telegraph signals nor a break-in signal will result in apremature operation of disconnect relay l.

Having thus interconnected operator line LO with the calling subscriberline Ll, the operator may new type out the usual challenge at theoperator station S0 to inform the subscriber at station Si that he is inconnection with the operator, the word operator, for example. By thisoperation, line L0 is opened and closed, as required. Each time the lineis opened, the cessation of current in the line winding of repeatingrelay RRO causes the armature of such relay to move from its illustratedmarking position into engagement with its spacing contact, therebyimpressing negative potential on spoke conductor SPO. Each time linecurrent is resumed in the line LO, the armature of relay RRO isreoperated from spacing position to marking position, therebydisconnecting the negative spacing potential and reconnecting conductorSP0 to the grids of tubes STD and BTO. At the end of the instant sendingoperation, relay RRO comes to rest in its illustrated marking position,in readiness for the receipt of instructions from the callingsubscriber.

Each time negative spacing potential'is placed on conductor SPO as abovenoted, such potential is transmitted through jack J O and the patchingcord in use (PCS), to spoke conductor SPl, by Way of jack J i or J I.With the armature of relay RR in marking condition, each application ofspacing current to conductor SP! results in the application of anegative biasing potential to the grids of tubes ST and BT, causing acessation of current through both tubes and consequently through theassociated subscriber line and the artificial line. When this occurs,the armature of relay RR is unaffected because it is still held inmarking position by current flow through its right-hand or biasingwinding. The resulting cessation of current flow over the subscriberline causes the usualresponse at the callingstation SI, wherea-t theoperators'challenge message is typed.

A3. Interconnecting The subscriber at substation SI may now type out thedesired instructions to the operator. It may be assumed that the callingsubscriber clesires that his line be interconnected with lines L2 andLL. In this event, the operator may telegraphically interconnect line L2with line Li through jacks J I and J2, and a cord such as PCI, and maythen employ a patching cord such as PCZ to interconnect line L2 withline LL through jacks J2 and JL. The concerned patching cords nowcomprise the hub of the interconnected system, the spokes being SP1,SP2, SPL, and SP0. It will be understood of course that as many, or asfew, lines as desired may be interconnected with the calling line byusing the above procedure.

Having made the interconnection, the operator may signal the subscriberat substation Si to proceed, as by typing a word such as start, atstation SO, resulting in the transmission of the correspondingcombination of spacing and marking signals over spoke conductor SP0, andthence by way of patching cord P03, to spoke conductor SPi. At this timespoke conductors SP2 and SPL are obviously energized by thespacingsignals, giving the same response at all interconnectedsubscriber lines. The response at repeater Rl 'is as describedhereinbefore, the current flow being stopped in each of the tubes ST andET for the duration of each negative spacing signal.

Having transmitted the proceed signal, the operator may withdraw .fromthe interconnection by removing the plugs of patching cord PC3 fromjacks J! and J0.

A4. Intercommum'cation The interconnected stations SI S2, and SL may nowintercommunicate as desired. On transmission from station SI, receivingrelay RR operates to spacing position on each interruption of line LIand returns to marking position on each reclosure of the line, therebytransmitting negative spacing signals over spoke conductor SP! andthence over the interconnecting hub (including the patching cords) tothe interconnected spoke conductors SP2 and SPL. It will be apparentthat a very large number of lines maybe thus interconnected foreifective intercornmunication without overloading the spacing contact ofany of the relays such as RR, because the only load imposed on suchspacingcontact is that represented by the current flow through the gridresistors of the repeaters such as R2 and RL, corresponding to gridresistor GR of the repeater RI. In practice, this grid resistor may beon the order of 100,000 ohms, or higher, up to a sub,- stantial fractionof a megohm, as it needs only to pass sufficient current to insure thequick resumption of positive potential on the control grids of theassociated tubes such as ST and BT upon disconnection of negativespacing potential from the interconnected spoke and hub conductors.

During message transmission, line tube LT in each line circuit respondsby terminating the current flow through the associated line relay 2 foreach spacing signal, and by reestablishing current flow therethrough foreach marking signal. For the most part, the spacing signals are tooshort for a slow-restoring relay of the nature of relayz torespond.

During transmission, the subscriber at any station may break in on thetransmitting station by opening the break key thereat, such as BKI, forabout one or two seconds. It may be assumed that station Si is thetransmitting station and that station S2 is the break-in station.Opening of line L2 causes the repeater. R2 to transmit spacing potentialover SP2 to the repeaters interconnected therewith. This transmission isbegun immediately if the break occurs during marking condition. If thebreak occurs during spacing condition, the receiving relay in repeaterR2 does not respond until the existing spacing condition is terminated.

In the repeater R! no current can flow through either the subscriberline or the artificial line during the time the negative markingpotential resulting from the break-in operation is impressed on thecontrol grids of tubes ST and BT, wherefore the apparatus at thetransmitting station Si temporarily ceases to function, therebyinforming the subscriber of the break. He is expected to ceasetransmission to permit his line Ll to remain closed for the receipt of amessage from the break-in station.

When the break key BKZ at the break-in station S2 is subsequentlyreclosed, the consequent resumption of current flow over'line L2 causesthe repeater R2 to remove spacing potential from the interconnected hubsystem, enabling transmission to proceed thereover from any station.

During transmission of the break-in spacing signal over the hub system,line relays 2 may be restored in all line circuits, and start the timingoperation of each of the associated timer tubes TT, but this does notcause disconnect signalling of the operator, for in the connectedcondition of any line, the associated transfer relay 3 is in operatedcondition to increase the length of the timing operation to about fiveseconds by including the associated resistor 8 in circuit with timingresistor l, as previously noted.

A5. Disconnect Signalling The subscriber at any interconnected stationcan signal the operator at any time to request that the entireconnection be taken down or that his line be disconnected from the otherline, or lines, of the interconnected group. Disconnect signalling isaccomplished by opening the break key such as BK! at station S! for aninterval exceeding the disconnect timing interval, assumed herein to befive seconds. Opening of the line causes spacing potential to be placedon the associated spoke conductor, whence it passes through the hubsystem to the other spoke conductors, causing a response at the linecircuit of all connected lines. In the line circuit 1.0!, the disconnectspacing potential on conductor SP! causes line tube LT to restore linerelay 2. Relay 2 disconnects the upper terminal of timing con denser 5and the grid of timing tube 'IT from negative charging potential andtransfers them to ground through resistors l and 8 in series (ICO'IItacts 2 of transfer relay 3 are open at this time because relay 3 isoperated through the upper contacts of J i or J I). At the end of aboutfive seconds, condenser E is discharged sufficiently to permit anoperative flow of current through timing tube TT, whereupon relay 4operates in a circuit from the associated alternating current terminalR, the winding of relay 4, front contact of armature l of relay 3 andthe said armature, plate of vtube TT, and thence to ground through the Ythe artificial line.

10 grounded cathode of such tube. At its contacts 2, relay 4 closes alighting circuit for disconnect lamp Di, while at its contacts i itcloses a selflocking circuit to ground through the front contact ofarmature 2 of transfer relay 33. Accordingly, relay 4 remains operatedfollowing reclosure of the break key at the signalling station and theconsequent reoperation of line relay 2 and cessation of current flowthrough timing tube TT.

Since the same line-circuit operation occurs at each interconnected line(each of the concerned disconnect lamps such as DI, D2 and DL .becomeslighted at about the same time), it may be necessary for the operator toinquire which station requires disconnection, or whether all stationsrequire disconnection. This may be accomplished by interconnecting theoperators equipment with the interconnected hub system as by way ofpatching cord PC3 and jacks JL' and J0. Upon learning what is required,the operator may temporarily take down the entire interconnection torestore the line circuits, and then reestablish such portion of it asmay be desired.

Upon removal of the plug from jack J l, transfer relay 3 restores,disconnecting and restoring disconnect relay 4 to extinguish lamp Di.Similar results occur at the other line circuits.

B. FIGURE 2 As previously noted, Fig. 2 shows a modification of theequipment associated with line Ll' of Fig. l in that differential linerelay 2-A replaces line tube LT and line relay 2. One winding of relay2A is in series with the associated subscriber line LI, and one windingis in series with Normally, relay 2-A is not operated because theopposed windings are of substantially equal strength, but it is operated(by current flow through its lower winding alone) when current flows inthe artificial line at a time when no current is flowing in theassociated subscriber line. This condition obtains only when theassociated station Si is transmitting. It does not obtain when stationSi is receiving, for at that time tubes ST and BT are both blocked, toprevent current flow through both lines, subscriber and artificial. Itmay be noted that, since line relay 2-A is normally restored instead ofnormally operated, the negative biasing potential is connected to theback contact of the relay armature, and the connection to timingresistors l and 8 is brought to the front contact of such armature. Inthe arrangement of Fig. 2, when one station of an interconnected pair orgroup transmits a disconnect signal (a prolonged line break) such signaloperates on no line circuit LCl-A except the associated one, andconsequently results in the lighting of the disconnect lamp, such as Di,only at the station transmitting the disconnect signal. Except as noted,the line circuit LCi-A is exactly similar to the line circuit LCI (Fig.l), and operation and restoration of relay 2-A produces results ashereinbefore described for restoration'and operation of line relay 2.When a disconnect signal is received, it suffices for the operator todisconnect the line over which it was received, it being unnecessary todisturb the connections to the remaining lines.

0. FIGURE 3 As previously noted, Fig. 3 shows a further modification ofthe equipment of line i (Fig. 1). Line circuit LCl-B is exactly similarto line cirright-hand or bias Winding thereof. rent flow through thiswinding is in such a direccuit LCl-A, except that the two windings ofline relay 2-13 are connected in aiding relationship rather than inopposed relationship, because operating current flows through only oneline (subscriber or artificial) at a time in Fig. 3.

Repeater Rl-B is generally similar to repeater RI of Figs. 1 and 2,except that phase-inverter tube PI has been added, and the grid circuit'of balance tube ET is controlled from the plate circuit of tube PIinstead of inparallel with the control grid of sending tube ST; More inparticular, the control grid of tube ST is connected to marking contactM of receiving relay RR, and is supplied with ground potential throughgrid resistor GR, as in Figs. 1 and 2. The position occupied in Figs. 1and 2 by the control grid of balance tube ET is occupied in Fig. 3 bythe grid of phaseinverter PI. Accordingly, current is normally flowin inthe plate-cathode circuit of tube ST and in the plate-cathode circuit oftube PI. Balance tube ET in Fig. 3 has its control grid normallysupplied with negative biasing potential through grid resistor GR,wherefore current flow is normally blocked through the plate-cathodecircuit of balance tube BT to provide a condition of no current in theassociated artificial line( including the middle winding of relay RR,the lower winding of line relay 2-13, and the associated adjustableresistor). It will be noted that the connecting resistor OR isinterconnected between the plate of phase inverter PI and the controlgrid of balance tube BT. Accordingly, cur.-

rent flows continuously from the positive plate of tube PI to thenegative control grid of balance tube BT. With no signal being receivedover conductor SP! at the repeater Ri-B, however, the flow of currentthrough resistor GR is very small because the positive voltagemaintained on the right-hand terminal of resistor CR is very low withtube PI in its normal conducting condition. Under this condition, thegrid resistor GR is of such value that it accepts all of the currentpassing through control resistor CR and'still maintains the potential ofthe control grid of tube BT sumciently negative to substantially blockthe flow of current through tube .BT and associated artificial line.

When transmission occurs from station SI (either to signal theswitchboard operator or to transmit signals to one or moreinterconnected stations), the opening of line Ll terminates the flow ofcurrent therein and through the lefthand winding of relay RR. Sincecurrent is normally not flowing in the artificial line, the only currentfiow remaining through the winding of relay RR is the normal currentflow through the The curtion as to cause the armature of relay RR tomove from marking position to spacing position. At this time, thecurrent fiow through tube PI is not affected, wherefore tube BT remains.unaffected.

When the associated line'Li is 'reclosed, the resultant resumption ofcurrent flow thereover and through the left-hand winding of receivingrelay RR causes the armature of such relay to return from its spacingcontact to its marking contact, thereby terminating the transmission ofspacing potential over the associated spoke conductor SP1.

With receiving relay RR in its illustrated marking position, receipt ofspacing potential over conductor SP1 from another'station causes theccntrol grid of tube ST and the grid of phaseinverter triode PI tobecome negative, thereby terminating the fiow of plate current in bothtubes. Tube ST directly terminates the current flow in the associatedline L1, as a spacing signal to stationSl. Asa result of cessation ofcurrent fiow through the plate-cathode circuit of phaseinverter PI, theplate element thereof tends to assume the full positive potential of thesupply source, thereby increasing'the flow of current from this positivesource through control resistor CR to the grid of balance tube As aresult, the potential drop across grid resistor GR is increasedsufilciently that the grid terminal thereof assumes a relativelypositive (or cathode) potential, rendering the plate-cathode circuit ofbalance tube BT conducting, whereupon current flows through theartificial line including the middle winding of relay RR and, the lowerwinding of line relay 2-13. The cessation of current flow in the platecircuit of tube ST and the starting of the flow of current through theplate circuit of tube BT occur substantially simultaneously, whereforethere is no open interval, when current is flowing through neither.Since the current normally flowing through the left-hand winding ofrelay RR isreplaced by a substantially equal current .fiow through themiddle winding thereof, and in the same effective direction, thearmatureof relay RR remains in engagement with its marking contact throughoutthe received marking signal.

When the negative spacing signal on the associated spokeconductor SP1 isterminated, current fiow through tubes PI and ST is resumed. Currentflow through the plate-cathode circuit of tube ET terminated as a resultof the lowerin of the positive plate potential at tube PI and theconsequent lessening of current fiow (in a positive sense) through theresistor CR to the control grid of tube BT. Upon the above-notedresumption of normal marking conditions in the repeater Ri-B,thearmature of relay RR is held in marking position by current fiowthrough the left-hand winding thereof instead of by current through themiddle winding thereof.

In the line circuit LCi-B, line relay 2-13 .is normally held in operatedcondition by current fiow through the upper winding thereof, in serieswith line Ll. The front contact of the armature of relay 'Z-B maintainsanegative potential on the grid of the associated tube TT and maintainstiming condenser 6 in charged condition. During the reception of aspacing signal at the repeater Rl-B, the cessation of current flow inthe upper winding of line relay 2-B is accompanied by the establishmentof the current flow through the lower winding thereof, in circuit withtube BT and the middle winding of relay RR. Consequently. line relay2-13 remains operated during the-reception of spacing'signals at therepeater Rl-B and theirtransmission over line Li to station Si. On theother hand, with the repeater RI-B in its normal marking condition, theopening of line L! at station Si pursuant to signal transmission causesa cessation of current flow through the upper winding of relay 2-3 with.no "compensating current flow in the lower winding thereof, under whichcondition relay 2-13 is currentless for the duration of the transmittedspacing signal. Being of the same type of slow-restoring relay as notedfor relays 2 and Z-A (Figs. 1'

and 2), relay 2-B may not respond to normal transmission. It doeshowever respond to the comparatively prolonged line breaks at break keyBK! of station Si, as when the associated line is opened for a periodsomewhat in excess of one second to signal the operator by a lighting ofthe associated call lamp CI, or is opened for an interval slightly inexcess of five seconds to signal the operator by a lighting of thedisconnect lamp DI at a time when a plug is in either of the jacks Jland JI'.

D. FIGURE 3A As previously noted, Fig. 3A shows a modification of therepeater Rl-B of Fig. 3, wherein the phase-inverter tube PI iseliminated and its phase-inverting function taken over by sending tubeST, which then becomes a dual-purpose tube.

In Fig. 3A, the armature of relay RR is normally held at rest in markingposition by current flow over the associated subscriber line (not shownin Fig. 3A) through the left-hand winding of the relay and throughsending tube ST. Current flow through the plate of tube BT andconsequently through the associated artificial line (including themiddle winding of relay RR) is normally prevented by the negative biasimposed on the control grid of balance tube BT through grid resistor GR.Control resistor CR is connected between the plate of tube ST and thegrid of tube BT, wherefore flow of current from the normally positiveplate of tube ST through resistor CR to the control grid of tube BTtends to impose a positive potential on the control grid of tube BT. Asnoted in connection with Fig. 3, the flow of current through resistor CRis normally ineffective to overcome the negative bias on the controlgrid of tube BT. because of the normal low positive potential on theright-hand terminal of resistor CR with the repeater in its normalmarking condition.

In Fig. 3A, the transmission of signals from. the associated subscriberlines does not cause current flow through the plate circuit of the tubeBT. because opening of the line disconnects the source of positivepotential from the plate of tube ST, permitting full negative potentialto reach the control grid of tube BT through grid resistor GR.

With the repeater of Fig. 3A in normal marking condition, the receptionof a negative spacing si nal over spoke conductor SPI renders thecontrol grid of sending tube ST negative. termimatin the current flowthrough the tube and over the associated line. When this occurs. theplate of tube ST assumes full positive potential, increasing the currentflow through control resistor CR sufiicien-tly to render the controlgrid of tube BT positive. As a result, plate current now flows throughthe tube BT and through the associated artificial line, including themiddle winding of relay RR. Current flow through this middle winding iseffective to maintan relay RR in its illustrated marking position. Whenthe negative spacing potential is subsequently removed from spokeconductor SPI, the control grid of tube ST resumes its normal negativepotential, impressed through grid resistor GR, whereupon the flow ofplate current through tube ST is resumed. The resultant lowering of thepositive potential on the plate of tube ST reduces the current flowthrough control resistor CR sufficiently to permit negative blockingpotential to be resumed at the control grid of balance tube BT. As aresult, current flow through the artificial line, including the middlewinding of relay RR, is terminated substantially simultaneously with theresumption of current flow through the subscriber line. Relay RR isthereby held continuously in marking condition. In Fig. 3A, thecomparativeily slight current flow through control resistor CR upon thereception of a spacing signal over conductor SH and its transmission tothe associated subscriber line by sending tube ST, is obtained over thesubscriber line, where fore the line current is not altogether broughtto zero. In practice, this small current will have practically no efiecton the operation of the equipment at the subscriber station whensuitable values are chosen for the negative biasing potential, gridresistor GR, and control resistor CR.

E. FIGURE 3B As previously noted, Fig. 3B shows a direct modification ofthe arrangement in Fig. 3A. In the arrangement in Fig. 3B, the currentfor charging the control resistor CR is obtained from the artificialline rather than from the subscriber line, thereby overcoming anytendency such as that above noted to interfere with the response of theequipment at the associated subscriber station.

In Fig. 3B, the arrangement is such that received spacing signals passfirst to the balance tube BT, which acts also as a phase-inverter tubeto cause such signals to pass in inverted sign to the sending tube ST,through which such signals are transmitted to the associated subscriberline. Because the phaseinverting means is interposed between spokeconductor SP! and sending tube ST, the spacing potential employed in asystem using the repeater modification of 3B is a ground, or cathode,potential, indicated in Fig. 33 as ground connected to the spacingterminal S of relay RR. The normal, marking potentital on conductor SPIis the negative blocking potential impressed thereon locally throughgrid resistor GR This negative potential normally blocks current flowthrough the plate of tube B1", wherefore the potential of the plate ofthis tube is normally substantially the full positive potential of thecurrent-supply source. The control grid of sending tube ST is suppliedwith a negative biasing potential through grid resistor GR, but thisnegative biasing potential is normally overcome by the current flow in apositive sense, supplied to the control grid of tube ST through thecontrol resistor CR, from the highly positive plate of the normallyblocked balance tube BT. As a result, normally the only current flowingthrough the balance line (including the middle winding of relay RR) isthe relatively slight current drawn by control resistor CR, While fullline current is normally flowing through the plate circuit of tube STand the left-hand Winding of relay RR'. This flow of line currentmaintains the armature of relay RR in its marking position against theinfluence of the weaker right-hand, bias winding.

When the associated line is opened for signal transmission, theresulting cessation of current flow through the left-hand winding ofrelay RR causes the armature of the relay to move from marking positionto spacing position, whereupon the normally negative potenfial of spokeconductor SPI is altered to ground potential as a spacing signal. Whenthe associated line is reclosed, the resulting resumption of currentflow through the tube ST and the left-hand Winding of relay RR causesthe armature of the relay to move 15 from its grounded spacing contactinto engag ment With its negative marking contact, terminating thespacing signal.

With the repeater arrangement of Fig. 3B in its normal markingcondition, the application of a spacing ground potential to spokeconductor SP1 by another similar repeater results in an altering of thepotential of the control grid of tube BT from its normally negativecondition to ground condition, whereupon full current fiows through theplate circuit of tube BT and over the associated artificial line,including the middle winding of relay RR. The esttblishment of thiscondition so lowers the normally high potential on the plate of balancetube BT that the reduced flow of current in a positive sense throughcontrol resistor CR to the control grid of tube ST permits such controlgrid to assume a negative blocking potential. As a result, the flow ofcurrent over the associated line, including the lefthand Winding ofrelay HR, is terminated, but relay ER is held in marking conditionduring this interval by the substitute fiow of current through themiddle winding thereof.

When the spacing ground potential is removed from spoke conductor SP Ithe control grid of tube BT resumes its normal negative blockingpotential, terminating the fiow of plate current through tube BT andreducing the fiow of current through the artificial line to the minutechargcontrol resistor CR to the control grid of tube ST, causingresumption of the flow of plate current through the later tube and overthe associated line. Relay HR is again held in marking position bycurrent flow through its left-hand Winding in the absence of the fiow ofsubstantial current through the middle winding thereof.

I claim:

1. In a telegraph repeating system, a plurality of telegraph lines and alocal interconnecting line, two-way telegraph repeaters interposedrespectively between said telegraph lines and said local line, anyrepeater including means controlled over the associated telegraph linefor impressing successive impulses of potential on said local line, afirst means in each other repeater responsive electrostatically to eachsuch impulse to impose a signal condition directly on the associatedtelegraph line, and a second means in each said other repeaterresponsive ele'ctrostatically to each last-named impulse to disable therepeater from responding to the imposed signal condition.

2. In a telegraph repeating system, a plurality of telegraph lines and alocal interconnecting line, two-way telegraph repeaters interposedrespectively between said telegraph lines and said local line, anyrepeater including means controlled over the associated telegraph linefor impressing suc cessive impulses of potential on said local line, andfirst and second means in each other repeater separately responsiveelectrostatically to each such impulse, said first means responding toimpose a signal condition directly on the associated telegraph line,said second means responding to directly disable the repeater fromrespending to the first means.

3. In a twoway repeater for telegraphically interconnecting a firsttelegraph line with a second telegraph line, a relay and a vacuum tubeconnected in series with the first line, a connection from the controlgrid of said vacuum tube 16 to the second line enabling the current fiowover the first line to be controlled according to the potential on thesecond line, and contacts controlled by said relay for disconnectingsaid control grid from the second line and for substituting a signalpotential.

4. In a two-way repeater for telegraphically interconnecting a firsttelegraph line with a second telegraph line, a vacuum tube connected inseries with the first line, a connection from the control grid of saidvacuum tube to the second line enabling the current flow over the firstline to be controlled according to the potential on the second line, andmeans controlled over the first line for disconnecting said control gridfrom the second line and for substituting a signal potential.

5. In a telegraph system, first line and a second line, sending meanscontrollable from the first line to send signals therefrom to the secondline, a sending tube in series with the first line having a control gridcontrollable from the second line to send signals therefrom to the firstline, and means including a second tube also having a control gridcontrollable from the second line to prevent said sending means fromresponding to the last-named signals.

6. In a telegraph system, a first line and a second line, a receivingrelay controllable from the first line to send signals therefrom to thesecond line, said relay including a line winding in series with thefirst line, a sending tube also in series with the first line having acontrol grid controllable from the second line to send signals therefromto the first line, a balance line including a balance winding of saidrelay, and a balance tube in series with said balance line having acontrol grid controllable from the second line to send signals therefromto the balance line to prevent said receiving relay from responding tothe signals sent by said sending tube.

7. In a telegraph system, a first line having a balance line associatedtherewith, a second line, a receiving relay controllable from the firstline to send signals therefrom to the second line, said relay includinga line winding in series with the first line and a balance winding inseries With the balance line, two tubes in series with the first andbalance lines respectively, each tube having a control grid controllableby signals on the second line to send signals to the associated line,the signals sent to the balance line preventing said receiving relayfrom responding to the signals sent to the first line, a common currentsource for energizing the first line and the balance line, and means forinverting the phase of the signals at one control grid with respecttothe signals at the other to alternate the demands of the two concernedlines for current from the common source.

8. In a telegraph system, a first line having a balance line associatedtherewith, a second line,- a receiving relay controllable from the firstline to send signals therefrom to the second line, said relay includinga line Winding in series with the first line and a balance winding inseries with the balance line, and tube means controlling current flowover the first line and over the balance line, said tube means includingcontrolgrid means controllable by signals on the second line to sendsignals to the first line and to the balance line, the signals sent tothe balance line preventing said receiving relay from responding to thesignals sent to the first line.

9. In a telegraph system, a first line having a balance line associatedtherewith, a second line,

17 a receiving relay controllable from the first line to send signalstherefrom to the second line, said relay including a line winding inseries with the first line and a balance winding in series with thebalance line, tube means controlling current flow over the first lineand over the balance line, said tube means including control-grid meanscontrollable by signals on the second line to send signals to the firstline and to the balance line, the signals sent to the balance linepreventing said receiving relay from responding to the signals sent tothe first line, a common current source for energizing the first lineand the balance line, and means for inverting the phase of the signalssent to one such line with respect to the signals sent to the other toalternate the demands of such lines for current from the common source.

ORLANDO BLYI-IOLDER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

